Session Initiation Protocol (SIP) is a protocol that allows a client to initiate, control and release a media session via a controlling server. It is possible for signalling using SIP to contain the media of the media session, for example instant messaging and presence information (information conveying a user's willingness and ability to engage in communications). However, more often, the media flows over a separate connection between a media server and the client, using for example Real-time Transport Protocol (RTP). There is no requirement for the media server and the SIP server to be physically located on the same hardware platform.
The SIP client includes a SIP client application that communicates with a SIP server application on the SIP server located elsewhere on a network. Each SIP server application is uniquely addressable through a combination of IP address and port to enable the SIP client application to contact the SIP server application.
Currently, SIP hardware endpoints are mainly designed to work with only a single application. A particular example is a VoIP (Voice over Internet Protocol) handset interacting with a VoIP PBX application.
SIP applications on SIP endpoints are assumed to be self-configuring, that is, there is no central authority to control what applications are loaded onto a SIP endpoint. Thus a SIP application may be loaded onto any SIP endpoint and executed as long as the application can spoof the application registration.
A SIP application may include multiple features. For example, in the case of the VoIP PBX application, features of the VoIP PBX application may include call park, call transfer, call waiting, voice mail, Do Not Disturb, etc.
There is no standard way that a particular SIP endpoint supporting multiple applications may have a subset of applications enabled, nor for an individual application to have a subset of features enabled.
As SIP has evolved the SIP hardware endpoints are being allowed more functionality and SIP servers are being designed to offer multiple applications.